The synthesis of water-soluble polymers and copolymers (polyacrylamide, acrylic acid-acrylamide copolymer, etc . . . ) is now the object of much research works. The mode of preparation used is generally invert emulsion polymerization. The final product is then in the form of a polymeric colloidal suspension dispersed in an oil, called an invert latex. This process makes possible a quick dissolution of the polymer in water, without formation of gels or aggregates as is the case when the product is packaged in powder form. It further facilitates storage and handling. However, a major drawback of the invert latices is their lack of stability which is reflected by a significant separation over time as well as by a wide size distribution of the particles.
This invention relates to a process of preparation of microlatices dispersed in a continuous oil phase by polymerization (or copolymerization) in a microemulsion of the water-in-oil type of at least one water-soluble vinyl monomer and the use of these invert microlatices in the absorption and the retention of aqueous fluids. In this process, the water-soluble monomer (or the mixture of monomers) is dissolved in water and dispersed in a mixture of surfactant and oil so that the proportions of components correspond to the monophase regions, also called microemulsions, of the monomer surfactant/oil/aqueous solution phase diagram. The monomer is trapped inside water-swollen micelles or else in the aqueous regions of a microemulsion of bicontinuous structure. These systems are optically transparent, thermodynamically stable and consequently lend themselves in particular to photochemical reactions. Such a process, known in particular by the teaching of patent FR-A-2,524,895, has been applied in particular to acrylamide, acrylic acid and N-vinylpyrrolidone. In this case, the water-soluble monomer is photochemically or thermally polymerized in microemulsion. The method consists in initiating the polymerization photochemically, for example, by ultraviolet irradiation, or thermally with an initiator that is hydrophobic, for example, azobisisobutyronitrile, or hydrophilic, for example, potassium persulfate.
The polymerization is photochemically performed very quickly, for example, in several minutes, quantitatively, and leads to the formation of microlatex whose particle radius is on the order of 15 nm. The surfactant used preferably is part of the class of anionic surfactants (such as sodium di-2-ethyl hexylsulfosuccinate) or of that of cationic surfactants (such as hexadecyl benzene-ammonium dimethyl bromide). It is also known by patent FR-A-2,565,592 to use, for the copolymerization of at least two acrylic monomers (for example (meth)acrylic acid and its alkaline salts), nonionic surfactants whose HLB (hydrophilelipophile balance) is from 8 to 11.
It is important to note that the two above-mentioned documents are concerned exclusively with the (co)polymerization of nonionic monomers (acrylamide, N-vinylpyrrolidone) or anionic monomers ((meth)acrylic acid and its alkaline salts) and that they do not indicate in any way that the processes of preparation of invert microlatices in a continuous oil phase by microemulsion polymerization of the water-in-oil type would be applicable in the case of cationic monomers. A first problem that this invention proposes to solve therefore consists in determining the conditions under which this type of process could be usefully applied in the (co)polymerization of cationic water-soluble vinyl monomers, if necessary mixed with at least one nonionic or anionic water-soluble vinyl monomer. A second problem that this invention aims to solve consists in determining the preparation conditions under which the microlatices obtained are stable (thermodynamically) and optically transparent. Still another problem that this invention proposes to solve consists in determining the conditions under which the invert microlatices thus obtained could be used for the absorption and the retention of aqueous fluids, also as additives (flotation and drainage additives) in the production of paper pulp.